1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device, and more particularly, to an LCD device and a method for fabricating the same, that maximizes light efficiency by compensating for the light transmissivity of each liquid crystal alignment domain of each pixel.
2. Discussion of the Related Art
The rise of the global information society has increased the demand for various display devices. Accordingly, many efforts have been made to research and develop various flat display devices such as liquid crystal display (LCD), plasma display panel (PDP), electroluminescent display (ELD), and vacuum fluorescent display (VFD). Different types of flat display devices have been developed as displays for various equipment.
Among the various flat display devices, liquid crystal display (LCD) devices have been most widely used due to advantageous characteristics of thin profile, lightness in weight, and low power consumption, making them an attractive substitute for Cathode Ray Tube (CRT) displays. In addition to mobile-type LCD devices for use in notebook computers, LCD devices have been developed for uses such as computer monitors and televisions.
Despite various technical developments in LCD technology having applications in different fields, research in enhancing the picture quality of the LCD device has been, in some respects, lacking as compared to other features and advantages of the LCD device. In order to use LCD devices in various fields as a general display, the key to developing LCD devices depends on whether LCD devices can provide a high quality picture, such as high resolution and high luminance with a large-sized screen, while still maintaining lightness in weight, thin profile, and low power consumption.
In general, an LCD device includes an LCD panel for displaying an image, and a driver for supplying a driving signal to the LCD panel. The LCD panel includes a first substrate and second substrate bonded to each other with a cell gap between them, and a liquid crystal layer formed between the first and second substrates.
The first substrate (TFT array substrate) includes a plurality of gate lines arranged along a first direction at a fixed interval; a plurality of data lines arranged along a second direction perpendicular to the first direction and at a fixed interval; a plurality of pixel electrodes arranged in a matrix-type configuration within pixel regions defined by the crossing of the gate and data lines; and a plurality of thin film transistors that apply video-signal voltages on the data lines to the pixel electrodes in response to gate signals on the gate lines.
The second substrate (color filter array substrate) includes a black matrix layer that blocks light from portions of the first substrate other than the pixel regions, an R/G/B color filter layer for displaying various colors, and a common electrode for producing the image.
The LCD device is driven according to the optical anisotropy and polarizing characteristics of the liquid crystal material. Liquid crystal molecules are aligned according to directional characteristics due to the liquid crystal molecules' long and thin shape. The alignment direction of the liquid crystal molecules of the liquid crystal layer is controlled by applying an electric field. Accordingly, light transmitted through the liquid crystal layer may be controlled by the alignment direction of the liquid crystal molecules, thereby displaying the image.
Active matrix-type LCD devices have been developed because of their high resolution and image quality, wherein the pixel electrodes are connected to the thin film transistors and are arranged in a matrix-type configuration.
A related art LCD device and a method for fabricating the same will be described with reference to the accompanying drawings.
FIG. 1 is a perspective view illustrating an LCD panel and a corresponding polarizing sheet in a multi-domain LCD device according to the related art. FIG. 2 illustrates an expected luminance distribution in each domain of an LCD device of FIG. 1.
As illustrated in FIG. 1 and FIG. 2, a related art multi-domain LCD device includes an LCD panel 10 and a polarizing sheet 25. The LCD panel 10 includes a plurality of pixel regions, each pixel region having a plurality of domains. In the polarizing sheet 25, a light-transmitting axis 26 is defined in one direction. The LCD panel includes first and second substrates opposite each other, and a liquid crystal layer 20 formed between the first and second substrates.
The polarizing sheet 25 is adhered to the outer surface of each of the first and second substrates. One polarizing sheet 25 having a light-transmitting axis 26 is formed on an upper surface of the LCD panel 10, and another polarizing sheet (not shown) having a light-transmitting axis is formed on a lower surface of the LCD panel 10. The light-transmitting axes of the polarizing sheets are formed at an angle of about 90° (or a predetermined angle) relative to each other.
After completing the array fabrication processes for the first and second substrates in the multi-domain LCD device according to the related art, a pretilt angle may be selectively applied to each domain by rubbing each of first and second alignment layers (not shown). Alternatively, in the array process, one pixel region may be divided into the plurality of domains through the design of the pixel electrode or common electrode in the pixel region of each of the first and second substrates.
FIG. 1 illustrates one pixel region with nine domains. In this case, the adjacent domains have the different alignment directions from one another, of which two may be referred to as a first domain 11a and a second domain 11b. 
Referring to FIG. 2, in the first domain 11a, the liquid crystal is aligned in the same direction as the light-transmitting axis 26 of the polarizing sheet 25. Accordingly, the light transmission in the first domain 11a is greater than the light transmission in the other domains. For the second domain 11b, the liquid crystal is aligned at the angle between 0° and 90° relative to the light-transmitting axis 26 of the polarizing sheet 25. That is, light transmission in the second domain 11b is relatively less than the light transmission in the first domain 11a. As a result, light leaks out from the first domain 11a in a black state.
Generally, the polarizing sheets are positioned on the lower and upper surfaces of the LCD panel. The light-transmitting axes of the two polarizing sheets may be positioned in parallel or at a relative angle of about 90°.
In case of a multi-domain LCD device having a plurality of pixel regions, and wherein each pixel region has a plurality of domains, the viewing angle is improved in the horizontal and vertical directions. However, it is difficult to improve the viewing angle in the diagonal direction. Accordingly, light leakage may be generated in the predetermined domain of the multi-domain LCD device, so that it is difficult to obtain the uniform light transmissivity.
An HAVA (high aperture vertical alignment) mode LCD device, which is a type of multi-domain LCD device, will be described as follows.
FIG. 3 is a cross sectional view of a high aperture vertical alignment HAVA mode LCD device according to the related art. FIG. 4 is a plan view of an HAVA mode LCD device according to the related art. FIG. 5 shows the alignment of liquid crystal in an HAVA mode LCD device of FIG. 4.
As illustrated in FIGS. 3-5, a related art HAVA mode LCD device includes first and second substrates 30 and 40, gate and data lines (not shown), a pixel electrode 35 and a dielectric protrusion 42. The first and second substrates 30 and 40 are positioned opposite each other. The gate lines (not shown) cross the data lines (not shown) at right angles, to define a pixel region. The pixel electrode 35 is formed on the pixel region of the first substrate 30. The dielectric protrusion 42 is formed on the second substrate 40 corresponding to with the center of the pixel region. In addition, an insulating layer 33 is formed between the gate line and the data line, between the gate line and the pixel electrode, and between the data line and the pixel electrode.
Further, a common electrode (not shown) is formed on an entire surface of the second substrate 40. On applying a voltage, a vertical electric field is formed between the pixel electrode 35 and the common electrode (not shown). A cell gap “d” is formed between opposite surfaces of the first and second substrates 30 and 40, and distance “I” defines the distance in a straight line between one side of the pixel electrode 35 and one outermost point of the dielectric protrusion 42.
In a HAVA mode LCD device, polarizing sheets are positioned on the rear surfaces of the first and second substrates 30 and 40.
Referring to FIG. 5, the dielectric protrusion 42 is formed corresponding to the center of the pixel region 50 in the HAVA mode LCD device. Accordingly, an electric field is formed in a radial direction from the dielectric protrusion 42, whereby the liquid crystal 53 is aligned in a radial direction according to the electric field.
As illustrated in FIG. 5, the unit pixel region is divided into a first domain 51a, a second domain 51b, a third domain 51c and a fourth domain 51d. In the first domain 51a, the liquid crystal is aligned in the horizontal direction. In the second domain 51b, the liquid crystal is aligned in the vertical direction. In the third domain 51c, the liquid crystal is aligned in a diagonal direction crossing from the lower left corner to the upper right corner. In the fourth domain 51d, the liquid crystal is aligned in a diagonal direction crossing from the lower right corner to the upper left corner.
The HAVA mode LCD device compensates for the liquid crystal aligned in the horizontal and vertical direction. That is, on applying a voltage to the HAVA mode LCD device, light leakage occurs at four corners of the pixel region (e.g., domains 51c and 51d), thereby deteriorating image contrast.
If the light-transmitting axes of the lower and upper polarizing sheets are twisted at an angle of 45°, the light leakage may be generated in the domains of liquid crystal aligned in the horizontal and vertical directions (e.g., domains 51a and 51b). To obtain the high aperture ratio in the multi-domain HAVA mode LCD device, even if the alignment process is performed in the pixel region of the LCD panel to obtain the various directions on alignment of liquid crystal, a light leakage may occur in certain domains, depending on the light-transmitting axis of the polarizing sheet.
Accordingly, related art LCD devices and method for fabricating them have the following disadvantages.
To obtain a high aperture ratio in the multi-domain LCD device including the HAVA mode of the related art, even if the alignment process is performed in the pixel region of the LCD panel to obtain the various directions on alignment of liquid crystal, the luminance differs by domain according to whether the liquid crystal of that domain is aligned in the same or different direction as the light-transmitting axis of the polarizing sheet. That is, the light leakage may occur in the black state, whereby the differences in luminance may be generated in the domains of one pixel region.